AL KAB (Iota Aurigae). Some bright stars do not get the respect
they deserve, in part because they have no well-known names,
because they carry middling Greek letter names, or are overwhelmed
by others in the constellations of great fame. So it is with
Auriga's Al Kab, short for Al Kab dhi'l
Inan, from Arabic meaning "the heel of the rein holder." The
name was originally applied to Gamma Aurigae, which actually is
more a part of Taurus, and is now always referred to as Elnath (Beta Tauri). Perhaps as a result,
Al Kab also carries the more modern moniker "Hassaleh." When we
look at Auriga, we more admire brilliant Capella, and gaze at the
triangle made by "the Kids," certainly not at Al Kab. Though at
bright third magnitude (2.69, but slightly variable) and fourth
brightest star in the constellation (third if you exclude Elnath),
Bayer still gave it the Greek letter "iota," preferring lower
letters for the Kids: Almaaz, Haedus I, and Haedus
II (Epsilon, Zeta, and Eta). At first glance, Al Kab is "just
one more orange class K (K3) giant." But one with a difference.
It is a "bright giant," more
luminous than most of the class. Its placement in the Taurus-Auriga part of
the Milky Way and a rather large distance of 510 light years also
causes it to be dimmed by roughly 0.6 magnitudes as a result of
interstellar dust absorption. Were the dust not present, Al Kab
would be a mid-second magnitude star. When the dust absorption and
infrared radiation from a 4390 Kelvin surface is taken into
account, the star is see to shine with a total radiance 5400 times
that of the Sun. With a then-calculated diameter 128 times that of
the Sun (60 percent the orbital size of
Earth), the star is so big that the angular diameter of its disk
can be measured (which gives a somewhat smaller radius of 106
solar). Seven or nine times the solar mass (depending on just how
the star is evolving), the dying orange giant is most likely fusing
helium to carbon in its deep core. It is also near the limit at
which stars explode as supernovae. If it
does not, it will die as a massive
white dwarf. Beginning
life a hot class B star similar to Graffias-1, it has
taken between 30 and 45 million years since birth to reach its
present state. Al Kab is also among the sky's brightest X-ray
producing "hybrid stars." Lesser giants produce X-rays from hot
coronae similar to the one found around the Sun, while among more
advanced giants, the coronae are replaced by cool, outward-flowing
winds. In Al Kab, we see both, which certainly elevates the star
to the status achieved by its constellation-mates. Thanks to Jeff
Bryan, who suggested this star.